Spacecraft’s journey to interstellar space helps put the solar system in perspective

It’s finally official: Voyager 1 has become the first human-made object to enter interstellar space, mission scientists report September 12 in Science. On August 25, 2012, the scientists say, Voyager 1 exited a giant invisible bubble called the heliosphere that is inflated by a torrent of subatomic particles spewing from the sun. Now the probe is surrounded almost exclusively by particles produced by other stars. But whether it’s correct to say that the probe has left the solar system depends on how you define the solar system. “From my perspective, Voyager is nowhere near the edge of the solar system,” says planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colo. The sun continues to exert gravitational dominance out to hundreds of times the distance of Voyager 1 from the sun, where trillions of icy pebbles, boulders and comets orbit. In the last 36 years, Voyager has traveled an impressive 25.4 billion kilometers, but it still has a long way to go to unambiguously depart the solar system.

OORT CLOUD
Distance from the sun: 5,000–100,000 AU
The sun, planets and Voyager probes sit inside the tiny yellow dot at right, within a giant sphere called the Oort cloud. This reservoir of trillions of ice chunks extends 100,000 astronomical units out, tethered to the sun by gravity. Astronomers believe these objects got thrown out of the inner solar system as the planets took shape 4.5 billion years ago. Occasionally these castaways pass near Earth: The comet ISON, which may light up the night sky this November, started out in the Oort cloud. The Voyagers would have to travel another 30,000 years before clearing this broadest definition of the solar system.

VOYAGER 1
Current distance from the sun: 126 AU
1 astronomical unit = 150 million
kilometers (Earth-sun distance)
Voyager 1 is now surrounded by a relatively thick fog of subatomic particles produced in the far reaches of the galaxy. Some particles originated in supernova explosions; others got blasted out of black holes. By 2016 astronomers expect the probe’s sibling spacecraft to pop through the solar bubble. Unlike Voyager 1, Voyager 2 carries a working instrument to measure the temperature and density of the interstellar medium. Both probes have enough plutonium power to communicate with Earth until about 2025.

HELIOPAUSE
Distance from the sun: about 122 AU
Until recently, Voyager 1 was traveling within the heliosphere, bathed in a thin mist of particles from the solar wind. Voyager 1 passed through the boundary between the heliosphere and interstellar space, called the heliopause, last August. But the border crossing was not cut-and-dried: Astronomers expected the magnetic field to change direction in interstellar space along with the particle population, yet the field has barely budged. Theorists are struggling to understand why.

TERMINATION SHOCK
Distance from the sun: about 90 AU
The solar wind gradually slows as it cruises past the planets. About 13 billion kilometers from where that wind originates, it slows down to about 350,000 kilometers per hour and generates a shock wave analogous to the one produced when a jet crosses the sound barrier. Voyager 1 reached this shock wave, known as the termination shock, in 2004. Beyond it, the solar wind wanes as the gateway to interstellar space approaches.

SOLAR WIND
The sun unleashes a continuous stream of subatomic particles at more than 1.5 million kilometers per hour. This solar wind permeates a radius of billions of kilo­meters in all directions and inflates the heliosphere. For some astrophysicists, the solar system is defined by the presence of the solar wind.

KUIPER BELT
Distance from the sun: 30–100 AU
For much of the past quarter century, Voyager 1 has been traversing this disk of icy objects (including Pluto) that were not incorporated into planets when the solar system formed.

TAIL
The sun, planets and entire heliosphere orbit the center of the galaxy at a brisk 83,000 kilometers per hour. In July NASA’s Interstellar Boundary Explorer satellite discovered that the sun drags behind it a cometlike tail of subatomic particles (not shown) that may stretch 10 times as far from the sun as Voyager 1’s current position. The finding shows that the solar bubble is shaped more like an elongated bullet than a sphere. Fortunately Voyager 1 trekked toward the leading edge of the bubble, where the distance to interstellar space is comparatively short.

THE PLANETS
Neptune’s distance from sun: 30 AU
The notion of the solar system as the sun plus eight planets (or nine, depending on your age) largely gets abandoned after grade school. Voyager 1 passed Neptune’s orbit in May 1987 and has since logged 14.2 billion kilometers.